Surface evolution during the mid-Proterozoic stalled by mantle warming under Columbia–Rodinia

Abstract

The thermal state of the mantle impacts lithospheric dynamics and Earth's surface evolution. Geodynamic simulations predict that aggregated continents may act as an effective thermal insulator leading to the warming of the underlying mantle (mantle warming hypothesis). Mantle warming weakens the continental lithosphere, which should lead to more distributed strain at plate margins, and result in lower-relief orogens, with lower metamorphic peak pressures (peak-P), and slower cooling and exhumation of metamorphic rocks. To test the mantle warming hypothesis, we determine the secular change of metamorphic cooling and exhumation rates based on a new dataset compiled from Neoarchean and younger orogens. We find that sluggish cooling and exhumation during the mid-Proterozoic (1.85–0.85 Ga; aka the boring billion) correlate with lower peak-P and higher thermobaric (temperature/pressure, T/P) ratios of metamorphic rocks. This result is consistent with a weakened continental lithosphere and the development of hotter lower-relief orogens under warmer mantle conditions during the mid-Proterozoic. We posit that the long-lived Columbia–Rodinia supercontinents may have acted as a thermal insulator that led to the mantle warming beneath the continental lithosphere and thus a distinctive style of mid-Proterozoic orogenesis. Low-relief mid-Proterozoic orogens with sluggish exhumation, in turn, reduced continental erosion and restricted delivery of bio-essential nutrients to the oceans, and decreased rates of organic carbon burial and thereby hindered oxygen accumulation in the atmosphere. The persistent nutrient deficiency in the oceans and suppressed atmospheric oxygenation may have stalled the surface evolution during the boring billion.